8" minimum in my eyes and prefer Wet Sounds, depending on what kind of boarding you do, if you surf more I would run either the Icon8, or the REV8, or if you do more wakeboarding I would do the REV8's or the REV10's.

Surface area rules in an open field environment and based on surface area one 8-inch speaker is nearly like having two 6.5-inch speakers. Plus, you will undoubtedly have more pod displacement with an 8-inch speaker and that determines the low frequency extension. So an 8-inch will be warmer and play deeper.
When it comes to midbass extension, a higher quantity of a smaller speaker will never compete with fewer of a larger speaker. This also makes a 10-inch tower speaker advantagous over an 8-inch speaker.